This invention relates to improvements in electrically-operated disc brake assemblies for vehicles of the kind in which a friction member is adapted to be applied to a rotatable brake disc by a brake-applying means including an electric motor which forms part of the assembly, and linear actuator means in the form of a screw device are adapted to apply an axially-directed brake-applying force to the friction member in response to operation of the motor.
A brake of the kind set forth is known from EP-A-0 339 024. In the brake of EP-A-0 339 024 the electric motor is used to drive an input to a reduction gearbox, and the gearbox is adapted to translate the high speed movement of the motor into a controlled high torque output to the screw device. In the brake disclosed in EP-A-0 339 024, the reduction gearbox typically has a ratio of 115:1. Such a high gear reduction is required in order to provide a suitable control of the clamp force from the high speed low torque, electric motor. Gearboxes on this type are by their nature very large and complex. The gearbox disclosed in EP-A-0 339 024 is of an epicyclic design having many individual components. The output from the gearbox is operatively connected to the screw device, disclosed in EP-A-0 339 024 as a ball screw
In an attempt to reduce the overall package size and the part count. with consequent reduction in mass and in order to provide a more compact assembly, it has been proposed in EP-A-0 275 783 to omit the gearbox and achieve all necessary reduction requirements by modifying the linear actuator to incorporate differential gearing. Specifically in EP-A-0 275 783 the motor is offset radially from the axis of the armature and drives a spur gear having a first power take-off, and a second power takeoff. Each power take-off is provided with a different number of teeth to drive co-operating members of the linear actuator at different speeds, thereby achieving relative movement between the components of the screw device. In this construction considerable radial space is required to accommodate the motor and the spur gear, and the drive mechanism is not incorporated in the actuator itself.
According to our invention in an electrically-operated disc brake assembly of the kind set forth for vehicles, the linear actuator comprises a screw mechanism adapted to translate rotation of the motor into axial movement of the friction member in a brake-applying direction.
Since a gearbox is omitted, inertia forces applied to the assembly are reduced, and the part count is also substantially reduced. We are also able to provided an assembly which is more compact, both axially and radially.
Preferably the screw mechanism comprises differential screws, and the use of very fine differential screws enables us to produce very fine pitch control, suitably 0.1 mm pitch.
The differential screws may comprise any convenient known arrangements. For example we can provide two-opposite hand threads on a single shaft, or the same hand threads on independent shafts. In another construction the differential screws may comprise concentric threading.
In a preferred construction a piston for applying the friction member to the disc works in a bore in a limb in a caliper, is keyed against rotation, and is internally screw-threaded, the armature of the electric motor is keyed to a drive member of differential outline having a first axially extending screw portion of smaller diameter screwed through a stationary nut, and a second axially extending screw-threaded portion of greater diameter in screw-threaded engagement with the piston, the threads on the first and second portions being of different pitch.
Preferably the pitch of the threads of the first portion is greater, suitably by 0.1 mm, than that of the threads of the second portion, whereby the resultant movement of the piston in an axial direction is equivalent to a pitch or ratio of 0.1 mm when the motor is rotated.
Preferably the armature of the motor is of cylindrical outline and surrounds the first portion, and the drive member has a longitudinal extending slot which slidably receives a diametrically extending key to permit movement of the drive member in an axial direction in response to rotation of the drive member by the motor.
When the caliper is of the single-sided reaction type, the piston is keyed to the friction member to prevent it from rotating, and the caliper moves bodily in an axial direction to apply a second friction member to the opposite face of the disc as the piston moves towards the disc in a brake-applying direction.
The screw-threaded engagements may be plain, ball screws, or roller screws.